In typical commercial reproduction apparatus (electrostatographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by application of heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly for typical reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
In the fuser assembly, at least one of the fuser and pressure rollers is compliant. Further, one of the fuser and pressure rollers, is driven by a driving motor (designated as the driving roller), and the other roller is driven by the friction in the nip (designated the driven roller). In the nip region, the compliant, uncompressible materials on the fuser and/or the pressure rollers are stressed and strained under pressure. This causes a phenomenon called “overdrive,” in which a segment along a linear element on the compliant surface of the roller in the nip region goes faster than a segment along a linear element on the free surface of the roller. The thicker the compliant, uncompressible material on a roller, the more the overdrive. The overdrive causes the receiver member being transported in the nip to go faster than the roller free surface speed.
Once the thickness of the compliant, uncompressible materials on the fuser and pressure rollers are determined, the factor that can affect the overdrive the most is the pressure applied to the fuser/pressure roller nip, or equivalently the nip width. In the process of fixing the toner image permanently to the receiver, it is sometime necessary to adjust nip width for different receiver member thickness, different image gloss level, and/or compensating temperature variation. This can cause varying receiver member speed through the fuser assembly. Because the reproduction apparatus process speed is usually fixed, this varying receiver member speed through the fuser assembly may cause speed mismatch with the surrounding reproduction apparatus sub-systems and components. The resultant speed mismatch problem may include pulling/pushing of the receiver member, distortion to the image and/or the receiver member, triggering false jam detection, or actually causing a jam. Therefore, there is a need to compensate for this speed variation at fusing station.